SUBPHYLUM CHELICERATA (chele, claw + ata, plural suffix)
The Chelicerata includes familiar animals like spiders, mites. and ticks. It also includes less familiar animals like horseshoe crabs and sea spiders. These animals have two tagmata.
(a) Prosoma or cephalothorax: Itis a sensory, feeding and locomotor tagma. It contains eye. But it has no has antennae. Paired appendages attach to the prosoma.
- Chelicerae: The first pair is called chelicerae. These are pincer like or chelate. These are used in feeding. These may are specialized as hollow fangs. These are also used for variety of other functions.
- Pedipalps: The second pair is called pedipalps. These are sensory. But these are also used in feeding, locomotion, or reproduction. Paired walking legs follow pedipalps.
(b)Opisthosonia: It is present posterior to prosoma. It contains digestive, reproductive, excretory and respiratory organs.
Members of merostomata are divided into two subclasses.
(a) Xiphosura: These are the horseshoe crabs.
(b) Eurypterida: These are the giant water scorpions. These
are extinct. The lived from the Cambrian (6oo million years ago) to the Permian period (280 million years ago).
Only four species of horseshoe crabs are living today. One species is Limulus polyphemus. It is 5.5 widely distributed in the Atlantic Ocean and the Gulf of Mexico.
Horseshoe scavenges on sandy and muddy substrates for annelids, small molluscs and other invertebrates. Their body form has remained unchanged for over 200 million years. The body of Horseshoe crab is composed of cephalothorax and opisthosoma.
1. Cephalothorax: A hard horseshoe shaped carapace covers the cephalothorax of horseshoe crabs. The chelicerae, pedipalps and first pairs of walking legs are chelate. These are used for walking and handling. Thelast pair of appendages has leaf like plates at ups. These are used for locomotion and digging.
2. Opisthosoma: It is composed of a long and unsegmented telson. Sometimes, wave action flips a horseshoe crab over. It arches its opisthosoma dorsally. It rolls to its side and flip right side up again. The first pair or opisthosomal appendages covers genital pores. These are called genital opercula. The remaining five pairs of appendages are book gills. These plates like gills resemble the pages of a closed book. So these are named as book gill. Blood circulates through gill and gases are exchanged between the blood and water. Horseshoe crab has an open circulatory system. Blood circulation of horseshoe crabs is similar to arachnids and crustaceans.
Reproduction: Horseshoe crabs are dioecious. Males and females congregate in intertidal areas. Male mounts the female. It grasps her with his pedipalps. The female digs shallow depressions in the sand. She sheds its eggs into depressions and male fertilizes them. Fertilized eggs are covered with sand. These eggs develop unattended.
CLASS ARACHNIDA (arachne, spider)
The majority of spiders, mites, ticks, scorpions and related forms are harmless. Even some are very beneficial to humans. Most zoologists believe that arachnids arose from the eurypterids. They were early terrestrial inhabitants. The earliest fossils of aquatic scorpions were found from the Silurian period (405 to 425 million vears ago). The fossils of the Terrestrial scorpions were found from the Devonian period (350 to 400 million years ago). Fossils of other arachnid groups are present by the Carboniferous period (280 to 345 million years ago).
Water conservation is a major problem for terrestrial organism. They have impermeable exoskeleton. This character is a preadapted ancestral character for terrestrial environment. Preadaptation is a structure present in members of specie which promotes reproductive success in new environmental situations.
Form and Function
Most arachnids are carnivores. They hold small arthropods with their chelicerae. Enzymes from the gut tract pour over the prey. Partially digested food is then taken into the mouth, Others inject enzyme into pay through hollow chelicerae (e.g.. spider). They suck partially digested an limit tissue.
Gut: The gut tract of arachnids is divided into three regions. The anterior portion is called foregut and the posterior portion is the hindgut. Both develop in foldings of the body wall. These are lined with cuticle. A portion the foregut is modified into a pumping pharynx. The hindgut is a site of water reabsorption. The midgut between the foregut and hindgut is noncuticular. It is lined with secretary and absorptive cells. Lateral diverticulae increase the area for absorption and storage.
Arachnids use coxal glands and Malpighian tubules for excreting nitrogenous wastes
1. Coxal gland: Coxal glands are paired, thin walled and spherical sacs. They are bathed in the blood of body sinuses. Nitrogenous wastes are absorbed across the wall of the sacs. These wastes are transported in a long, convoluted tubule. These are finally excreted through excretory pores present at the base of’ the posterior appendages.
2. Malpighian tubules: Some arachnids are adapted to dry environment. They posses malpighian tubules of excretion. Malpighian tubules are blind ending diverticulae of the gut tract. These arise at the juncture of the midgut and hindgut. These tubules absorb waste materials from blood and open into the gut tract. Excretory wastes are then eliminated with digestive wastes. The major excretory product of arachnid is uric acid. The release of uric acid has advantage for terrestrial animals. It is excreted as a semisolid with little water loss.
Gases are exchanged with minimal water loss. The arachnids ha‘ e few exposed respiratory surfaces. Two structures are present in arachnids for respiration:
1. Book lungs: Some arachnids possess book lungs. Book lungs are modification of the book gills of Merostomata. Book lungs are paired imagination of the ventral body wall. These invaginations folds into series of leaf like lamellae. Air enters the book lung through a lit like opening. It circulates between lamellae. Respiratory gases diffuse between the blood moving among the lamella and the air in the lung chamber.
2. Trachea: Other arachnids possess tubule systems are called trachea. Trachea is a series of branched, chitin lined tubules. These tubules deliver air directly to body issues. These trachea open to the outside through openings called spiracles. Spiracles are present along the ventral or lateral sides of abdomen.
The arachnids have open circulatory system. Dorsal contractile vessel or hearts pump blood into tissue spaces. The coelom is reduced to cavities in arthropods. These cavities are surrounded by gonads and coxal glands. Large tissue spaces, or sinuses are derived from the blastocoel. These are called hemocoel. Blood bathes the tissues and then returns to the dorsal aorta through openings in the aorta called ostia. Arachnid blood contains dissolved respiratory pigment hemocyanin. It also possess amoeboid cell. ‘these cells help in clotting and body defense.
The nervous system of all arthropods is ventral. The nervous system of arachnids is centralized except scorpions. It is formed by the Fusion of ganglia.
Sense organs: The body of an arachnid has a variety of sensory structures. Most mechanoreceptors and chemoreceptor are modifications or the exoskeleton. These are present in the form of projections, pores, and slits, together with sensory and accessory cells. Collectively, these receptors are called sensilla. There are following type of sensilla in arachnids:
1. Setae: setae are hair like cuticular modifications. These may be present in the form of membranous sockets. Displacement of setae initiates a nerve impulse in an associated nerve cell.
2. Vibration receptors: Vibration receptors are very important to some arachnids. Spiders use webs to capture prey. It determines size of the insect and its position on the web by the vibrations.
3. Chemoreceptors: Arachnids can taste and smell chemical. Small pores in the exoskeleton are present in arachnids. These are associated with peg like or other modifications or the exoskeleton. They allow chemicals to stimulate the nerve cells.
4. Eyes: Arachnids possess one or more pairs of eyes. They use eyes primarily for detecting movement and changes in light intensity. The eyes of some hunting spiders can form images.
Arachnids are dioecious. Paired genital openings are present on the ventral side of the second abdominal segment. Sperm transfer is indirect. The male packs sperm in a spermatophore. It is then transferred it to the female. The male confirms the female of same specie by courtship. Courtship also attracts a female to the spermatophore. It also determines the position the female to receive the spermatophore. In some taxa (e.g.. spiders) copulation occurs. Sperm is transferred through a modified pedipalp of the male. Development is direct. The young hatch from eggs as small adults. Manyarachnids tend their developing eggs and young during and after development.
Members of the class Pycnogonida are the sea spiders. All are marine. They are most common in cold waters. Pycnogonida live on the ocean floor. They feed on cnidarian poi is and ectoprocts. Some sea spiders feed by sticking prey tissues through a proboscis. Others tear prey with their chelicerac.
Reproduction: Pycnogonids are dioecious. Gonads are U-shaped. branches of the gonads extend into each leg. Gonophores are present on one of the pairs of legs. The female release the eggs and the male fertilize them. The fertilized eggs form spherical masses. They are attached to the pair of elongated appendages of male called ovigers. They are brooded there until hatching.